Dielectrophoresis (DEP) relates to the physical phenomenon whereby neutral particles, when subject to nonuniform, time stationary (DC) or time varying (AC) electric fields, experience a net force directed towards locations with increasing (pDEP) or decreasing (nDEP) field intensity. If the intensity of the said dielectrophoretic force is comparable to the gravitational one, an equilibrium may be established in order to levitate small particles. The intensity of the dielectrophoretic force, as well as its direction, strongly depend on the dielectric and conductive properties of particles and on the medium in which the body is immersed. In turn, these properties may vary as a function of frequency for AC fields.
A description of the theory of dielectrophoresis has been published by H. A. Pohl in “Dielectrophoresis” Cambridge University Press (Cambridge 1978). A theoretical formulation of a case of particular interest is reported in Biochimica et Biophysica Acta 1243 (1995) p. 185-194, and Journal of Physics, D; Applied Physics, 27 (1994) pp. 1571-1574.
Studies on the action of dielectrophoresis on both biological matter (cells, bacteria, viruses DNA, etc.) and inorganic matter particles have lately proposed using DEP forces for the isolation of elements from a mixture of microorganisms, their characterization by differences in physical properties and their general manipulation. For such purposes, the suggestion has been to utilize systems of the same scale of particle size, in order to reduce the potentials required by electrical field distributions.
U.S. Pat. Nos. 5,888,370, 4,305,797, 5,454,472, 4,326,934, 5,489,506, 5,589,047, 5,814,200, teach different methods of separating particles in a sample, based on differences in dielectric and conductive properties characterizing the species they belong to. The main drawback, common to all devices proposed resides in the requirement of mechanical and fluid dynamic microsystems for moving fluids within the system. Moreover, each apparatus of the above listed patents involves contact and friction of particles with the surfaces of the system, compromising their mobility and integrity.
U.S. Pat. No. 5,344,535 teaches a system for the characterization of microorganism properties. The disclosed apparatus and the proposed method have the shortcoming of providing data on a large number of bodies, lacking the advantages of analysis on a single particle. In addition, the disclosed system is unable to prevent contact of particles with device surfaces.
U.S. Pat. No. 4,956,065 teaches an apparatus to levitate single particles and analyze their physical properties. However, this device requires a feedback control system since it employs pDEP. Moreover, the system is unsuitable for miniaturization, having a three-dimensional topology which is not compatible with mainstream microelectronic fabrication technologies.
The paper by T. Schnelle, R. Hagedorn, G. Fuhr, S. Fiedler, T. Muller in “Biochimica et Biophysica Acta”, 1157(1993) pp. 127-140, describes research and experiments on the creation of three-dimensional potential cages for the manipulation of particles. However, the proposed structures are very difficult to fabricate in scale with the size of cells (required for trapping a single cell in the cage). In fact, the major problem of these systems is the vertical alignment of two structures on a micro-metric scale.